PPM application

Configure the PPM application to read RC-receiver pulse-width signals and drive the motor.

The PPM (Pulse Position Modulation) application reads servo-style pulse-width signals produced by RC receivers and standard ESC throttle inputs. It decodes the pulse width into a normalised level and applies it to the motor using the configured control type.

Signal specification

Default pulse-width range as defined in appconf_default.h:

Parameter	Default value	Description
pulse_start	1.0 ms	Minimum pulse width (full reverse or idle)
pulse_end	2.0 ms	Maximum pulse width (full throttle)
pulse_center	1.5 ms	Center/neutral pulse width
hyst	0.15	Deadband around center (normalised 0–1)

These map to the ppm_config struct fields pulse_start, pulse_end, and pulse_center.

Most RC receivers and flight controllers output a standard 1–2 ms pulse at 50 Hz. VESC can read this directly on the PPM input pin.

Control types

The ppm_control_type enum determines how the decoded pulse level controls the motor:

typedef enum {
    PPM_CTRL_TYPE_NONE = 0,
    PPM_CTRL_TYPE_CURRENT,
    PPM_CTRL_TYPE_CURRENT_NOREV,
    PPM_CTRL_TYPE_CURRENT_NOREV_BRAKE,
    PPM_CTRL_TYPE_DUTY,
    PPM_CTRL_TYPE_DUTY_NOREV,
    PPM_CTRL_TYPE_PID,
    PPM_CTRL_TYPE_PID_NOREV,
    PPM_CTRL_TYPE_CURRENT_BRAKE_REV_HYST,
    PPM_CTRL_TYPE_CURRENT_SMART_REV,
    PPM_CTRL_TYPE_PID_POSITION_180,
    PPM_CTRL_TYPE_PID_POSITION_360,
} ppm_control_type;

Control type	Description
CURRENT	Bidirectional current control. Below center = reverse.
CURRENT_NOREV	Current control, forward only.
CURRENT_NOREV_BRAKE	Forward current with braking below neutral.
DUTY	Direct duty-cycle control, bidirectional.
DUTY_NOREV	Duty-cycle, forward only.
PID	Speed PID, bidirectional.
PID_NOREV	Speed PID, forward only.
CURRENT_SMART_REV	Current control with timed smart-reverse.
PID_POSITION_180	Position PID over ±180°.
PID_POSITION_360	Position PID over 0–360°.

Configuration parameters

The full ppm_config struct:

typedef struct {
    ppm_control_type ctrl_type;
    float pid_max_erpm;
    float hyst;
    float pulse_start;
    float pulse_end;
    float pulse_center;
    bool median_filter;
    SAFE_START_MODE safe_start;
    float throttle_exp;
    float throttle_exp_brake;
    thr_exp_mode throttle_exp_mode;
    float ramp_time_pos;
    float ramp_time_neg;
    bool multi_esc;
    bool tc;
    float tc_max_diff;
    float max_erpm_for_dir;
    float smart_rev_max_duty;
    float smart_rev_ramp_time;
} ppm_config;

Key parameters

Field	Default	Description
median_filter	true	Applies a 3-sample median filter to reduce glitches.
safe_start	SAFE_START_REGULAR	Requires throttle to return to neutral before output begins.
ramp_time_pos	0.4 s	Time to ramp from 0 to full positive output.
ramp_time_neg	0.2 s	Time to ramp from 0 to full negative output.
multi_esc	true	Forwards the command to other VESCs on the CAN bus.
tc	false	Enables traction control between CAN-connected ESCs.
tc_max_diff	3000 ERPM	Maximum allowed ERPM difference between ESCs for traction control.
smart_rev_max_duty	0.07	Maximum duty cycle during smart reverse.
smart_rev_ramp_time	3.0 s	Time the stick must be held in reverse before smart-reverse activates.

Runtime API

void app_ppm_start(void);
void app_ppm_stop(void);
void app_ppm_configure(ppm_config *conf);
float app_ppm_get_decoded_level(void);
void app_ppm_detach(bool detach);
void app_ppm_override(float val);

app_ppm_get_decoded_level returns the current decoded input in the range −1.0 to 1.0. app_ppm_detach and app_ppm_override let other firmware components take temporary control of the output without stopping the PPM thread.

Safe start

With safe_start enabled, the motor will not respond until the PPM signal returns to the neutral (center) position after power-on or a timeout. This prevents unintentional movement when power is applied with the throttle open.

The firmware requires a minimum of MIN_PULSES_WITHOUT_POWER (50) valid pulses at neutral before allowing output. This constant is defined at the top of app_ppm.c.